The invention disclosed herein is an improvement to the inventor's prior automatic pellet mill controllers as disclosed and claimed in U.S. Pat. Nos. 3,932,736, 4,340,937 and U.S. Pat. No. 4,463,430, all of which were invented by the inventor herein and which are commonly owned by the assignee of the present invention. These prior patents are incorporated herein by reference.
The automatic pellet mill controllers disclosed in these prior patents essentially sense the temperature of the material as it is processed by the pelleting apparatus and at various points therealong, measures the difference between two of those sensed temperatures, and controls the operation of the pelleting apparatus by controlling the flow of ingredients therethrough. Of course, each of these systems has its own unique features and reference should be made directly to those prior patents for complete details of each system.
The temperature of the material is sensed at various points in the pellet apparatus through the use of direct contact temperature sensors, such as 106 or 109 as shown in the '937 patent which requires direct physical contact between the material and the temperature sensor to achieve an accurate measurement thereof. In the prior art, temperature sensors are available with stainless steel dual walled shielding to withstand the abrasive effect of the granular material as it rubbed against the temperature sensor. Although these temperature sensors do perform adequately, they represent an on-going maintenance requirement requiring periodic checking and replacement thereof to ensure continuous, satisfactory pelleting. Furthermore, at lower levels of production, there is an opportunity for air to partially surround the probe instead of material because there is an insufficient mass of material flowing through the system. This results in some inaccuracies in temperature measurement which, if not accounted for, can detract from the ability of the control to maximize production throughput and pellet quality.
To solve these and other difficulties encountered with using direct contact temperature probes in the prior art, the inventor herein has succeeded in adapting a non-contacting, IR sensing temperature sensor to the pellet mill controllers which sense the temperature of the material and use that parameter in controlling the pellet mill. The inventor has selected a Raytec.TM. Thermalert II.TM. Model No. T2L2 which senses the infrared frequencies emanating from the material to obtain a very accurate temperature reading of the material. Replacement of the direct contact temperature sensors of the inventor's prior systems, or for that matter other systems, eliminates the problems experienced therein. Additionally, the inventor has also discovered that there are advantages to sensing the temperature of the rotating die within the mill and using that temperature in combination with the temperature of the material as it enters the mill, much as in the manner of the .DELTA.T mill control disclosed in the '937 patent mentioned above. When the mill is first started up, the die is cold, and its temperature is unknown until pellets are formed by the die and their temperature measured. Therefore, directly sensing the temperature of the die gives a direct indication of the temperature at which the first pellets will be formed, and will also directly follow the heating up of the die as the pelleting run continues. This provides for a smoother start-up of each pelleting run, and more accurate control of the pelleting process to produce good quality pellets even at the beginning of a run.
Still another advantage in using a noncontacting temperature probe to measure the temperature of the die is that pellets are formed by material being forced through the holes in the die so that the temperature of the pellet as it is formed is almost exactly the same temperature as that of the die. This pellet forming temperature is one of the critical parameters to be measured and controlled to achieve uniform pellets of good quality during a pelleting run. While the direct contact temperature probe utilized in prior systems does sense the temperature of the pellets shortly after leaving the die, there is an inherent advantage in sensing the temperature of the die which is the temperature of the pellet immediately as it is formed. This thereby eliminates any error from a variation in temperature of the pellet as it moves from the die to the direct contact temperature probe in the prior art.
The foregoing has been a brief summary description of some of the principal advantages and features of the present invention. A greater understanding and appreciation for further details of the invention may be obtained by referring to the drawings and description of the preferred embodiment which follow.